Measurements of the water potential of plant tissue and nonliving samples generally are made with thermocouple psychrometers by enclosing the sample together with a thermocouple in a small container kept at a constant temperature and determining the degree of cooling of the thermocouple as water evaporates from it and is absorbed by the sample. It can be demonstrated that the rate of vapor transfer is proportional to the difference in potential between the thermocouple and sample.
The isopiestic technique is a variant of the above psychrometer technique and uses the psychrometer principle to find a solution which neither loses nor gains water from the sample. The potential of the solution, which is known, is then equal to the potential of the sample. Certain corrections are applied to the measurements and have been shown to be valid, thus allowing the method to give absolute rather than relative measures of water potential; that is, the water potential of a sample having a known potential is the same as the water potential indicated by the instrument. The isopiestic method is the only method developed so far that has been shown to give absolute values of water potential.
To find the solution that neither gains nor loses water from the sample, a solution of known vapor pressure is placed on a thermocouple and sealed together with the sample in the chamber. The thermocouple measures solution temperature and, when the first reading is completed, the solution is replaced with a second solution of different vapor pressure and the reading is repeated. The solution that exchanges no vapor with the sample has the same vapor pressure and thus the same water potential as the sample, i.e., the system is isopiestic and the thermocouple output is zero. In practice, it is not necessary to use a solution that is exactly isopiestic because the output of the thermocouple is linearly proportional to the difference in vapor pressure between each solution and the sample, and the output of the thermocouple can be extrapolated to the isopiestic point.
The advantages of the isopiestic technique are that it is
(1) the only technique requiring no calibration, PA1 (2) the only psychrometer method in which the diffusive characteristics of the sample do not affect the determination, and PA1 (3) the only psychrometer method that can measure water potentials higher than -2 bars. PA1 (1) the materials are selected to have a minimum thermal activity (small voltage/temperature coefficient) and PA1 (2) temperature gradients are minimized around all nanovolt parts of the system.
Thus determination is simplified, more accurate, and able to resolve a wider range of water potentials than other techniques.
The measuring of plant water status by the isopiestic technique is disclosed in the article entitled "Isopiestic Technique for Measuring Leaf Water Potentials with a Thermocouple Psychrometer", Proceedings of the National Academy of Sciences, U.S.A., Vol. 54, October, 1965, pp. 1044-1051. The procedure of vapor pressure measurement by liquid on a thermocouple is disclosed in the article entitled "Thermocouple for Vapor Pressure Measurement in Biological and Soil Systems at High Humidity", Science, Vol. 120, Oct. 31, 1958, pp. 1089-1090. The construction and performance of a welded wire, wet loop, thermocouple psychrometer measuring junction are disclosed in an article entitled "Construction of Welded `Wet Loop` Thermocouple Psychrometer Junctions", Agronomy Journal, Vol. 66, May-June, 1974, pp. 456-457.
Previously known methods of constructing instruments for the isopiestic technique are detrimental to the technique. In particular, thermocouple construction involving difficult soldering creates extraneous thermal activity at the junctions and requires much testing before use. Normal thermal activity of electrical junctions interfere with the isopiestic technique and must be minimized.
Presently there are no switching systems for isopiestic thermocouples capable of automatically switching a plurality of thermocouples that have the low thermal activity that is important to this technique.
It is an object of the present invention to provide an improved thermocouple and an isopiestic technique that (1) has low enough thermal activity at the electrical junctions of the equipment to avoid interference with the test measurements, and (2) has an improved thermocouple that is simple enough to make commercial production feasible.
It is an object of this invention to provide a thermocouple for measuring by the isopiestic technique that supports a substantially more effective object for testing.
It is a further object of this invention to provide a sturdy thermocouple for measuring by the isopiestic technique.
Another object of this invention is providing readings from any of a plurality of thermocouples for measuring by the isopiestic technique by using a switching system for selecting operation of the thermocouples having relays with thermovoltage of the order of .+-. 5 nanovolts.